fix($compile): properly handle setting srcset to undefined

Previously, calling `Attributes#$set('srcset', value)` on an `<img>` element would throw if `value`
were undefined, as it assumed `value` is always a string.
This commit fixes the issue, by skipping the unnecessary string manipulation when `value` is not
defined.

Closes #14470

Closes #14493

src/ng/compile.js

@@ -1447,7 +1447,7 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
             (nodeName === 'img' && key === 'src')) {
           // sanitize a[href] and img[src] values
           this[key] = value = $$sanitizeUri(value, key === 'src');
-        } else if (nodeName === 'img' && key === 'srcset') {
+        } else if (nodeName === 'img' && key === 'srcset' && isDefined(value)) {
           // sanitize img[srcset] values
           var result = "";
 

test/ng/compileSpec.js

@@ -9394,6 +9394,27 @@ describe('$compile', function() {
 
   describe('img[srcset] sanitization', function() {
 
+    it('should not error if undefined', function() {
+      var linked = false;
+      module(function() {
+        directive('setter', valueFn(function(scope, elem, attrs) {
+          attrs.$set('srcset', 'http://example.com/');
+          expect(attrs.srcset).toBe('http://example.com/');
+
+          attrs.$set('srcset', undefined);
+          expect(attrs.srcset).toBeUndefined();
+
+          linked = true;
+        }));
+      });
+      inject(function($compile, $rootScope) {
+        element = $compile('<img setter></img>')($rootScope);
+
+        expect(linked).toBe(true);
+        expect(element.attr('srcset')).toBeUndefined();
+      });
+    });
+
     it('should NOT require trusted values for img srcset', inject(function($rootScope, $compile, $sce) {
       element = $compile('<img srcset="{{testUrl}}"></img>')($rootScope);
       $rootScope.testUrl = 'http://example.com/image.png';

test/ng/directive/ngSrcsetSpec.js

@@ -28,5 +28,10 @@ describe('ngSrcset', function() {
     $rootScope.$digest();
     expect(element.attr('srcset')).toBe('http://example.com/image1.png 1x,unsafe:javascript:doEvilStuff() 2x');
   }));
+
+  it('should not throw an error if undefined', inject(function($rootScope, $compile) {
+    element = $compile('<img ng-attr-srcset="{{undefined}}">')($rootScope);
+    $rootScope.$digest();
+  }));
 });